Pain behind one knee

P
I have been having a throbbing pain behind my left knee. It happened a few weeks ago and again today - my veins are pretty blue and visible behind both knees.
Not sure if it's just a varicose vein thing ? Or if I could have a blood clot.
I have been on airplanes 4 times since being pregnant but have always worn compression socks or leggings
Anyone else have this pain?